1. Field of the Invention
The present invention pertains to techniques for detecting acoustic disturbances. More particularly, the present invention is related to methods and apparatus for detecting noises, and for processing electrical signals produced by noise detectors to convey information about the detected noise. The present invention finds particular application in the field of well logging.
2. Description of Prior Art
Various techniques are known for logging wells wherein one or more logging tools, designed to detect or measure various characteristics of underground formations or of fluid in the well, are positioned in the well at various depths where such information is to be gathered. Typically, a logging run of a well is carried out by lowering a logging tool within a well to at least the greatest depth at which information is to be gathered, and then raising the tool throughout the region under investigation while the tool is operated to sense the particular characteristics to be observed. Such maneuvering of the tool is generally done with the tool suspended and operated by means of a wireline, which may also include one or more electrical conductors by which operating instructions may be conveyed from the surface to the tool, and by which data signals may be transmitted from the tool to the surface for processing.
One technique for logging wells employs the use of a noise tool whereby downhole sounds may be detected and, at the surface, analyzed electronically to determine, for example, the locations of fluid leaks into or out of the well borehole. Noise tool logging may be carried out generally in uncased as well as cased wells. The movement of fluid between a formation and the borehole by way of a fissure or other aperture in the formation face at the well is generally accompanied by noise including high frequency sounds. Similarly, gas rushing through a puncture or perforation in a casing will generate a noise spectrum including high frequencies. Movement of liquids in similar fashions may be accompanied by noise, including possibly high frequency sounds. The movement of fluid behind a casing and from one formation to another will also generate noise. However, prior noise tools have been limited in their usage for several reasons. Movement of a logging sonde within a well generates considerable noise, particularly in the audio frequencies, which tends to drown out the noise generated by fluid movements that are to be detected. Consequently, prior noise tools have generally been used by positioning and holding the tool at a specified location, with all other operations stopped, to carry out the noise-measuring process. The tool is then moved to another location for further data gathering, if necessary, with no data being acquired during the movement. All measurement are thus made with the tool in a stationary configuration.
Further, data from prior noise tools have typically been analyzed in extended frequency ranges. For example, with the acoustic disturbance sensor in the noise tool being a stack of piezoelectric crystals, the electronic signals produced by the sensor are processed through a collection of high pass filters to yield a like number of broad frequency spectrum signals whose amplitude versus depth values are compared in the quest for locating leaks, for example. Consequently, the data has been used in analog form. Since the electronic signals from the sensor must be carried to the surface for analysis by means of a wireline, large signal attenuation is experienced in the analog, amplitude modulated signals, particularly in the case of high frequency signals. The problem becomes acute, especially in the high frequency ranges, for deep wells so that, at the surface, the high frequency signals are essentially lost. It is desirable to provide a technique for logging a well by detecting the presence of high frequency sounds, wherein the high frequency sounds may be sensed with sufficient sensitivity, and the corresponding information signals transmitted to the surface without undue attenuation, and wherein the noise measurements could be made while the logging tool is being moved along the well bore.
Piezoelectric sensors in prior noise tools generally include a plurality of crystals that are tubular in form. The crystals are arranged in a longitudinal stack around a central core. The radially inward and outward surfaces of the crystals may be plated with silver for purposes of electrical contact with conductors which carry the electrical signals generated by the piezoelectric stack in response to acoustic disturbances incident on the stack. The inner surfaces of all of the crystals are electrically connected together by means of springs positioned within the core and held under compression between the radially inward surfaces of the crystals and a central conducting rod contained within the core, which is otherwise constructed of insulating material. The radially outward surfaces of the crystals are connected together by means of a braided wire which is soldered to the silver plating on the crystal surfaces. The crystals are longitudinally mutually separated by O-rings acting as spacers and insulators. The braided wire passes over the O-rings. In this way, the crystals in the stack are all connected in parallel, so that the electrical signals generated by the stack of crystals is the average of the electrical signals produced by all of the individual crystals acting alone in response to the same acoustic disturbances.
The piezoelectric stack is enclosed within a cylindrical diaphragm which is generally a thin-walled stainless steel tube. The tube is filled with oil and, when acoustic disturbances are incident on the diaphragm, the disturbances are transmitted through the thin wall of the diaphragm and the oil to the crystals. However, the braided wire may contact the vibrating diaphragm in the vicinity of an O-ring where the braided wire is positioned particularly close to the diaphragm. Such contact with the diaphragm provides a ground path for the conductor, and generates large amounts of electronic noise which interferes with the signal. Further, it may be difficult to solder the braided wire to the silver plate, with the plate at that point possibly detaching from the crystal surface. Finally, with the crystals thus joined together by means of the braided wire to which the crystals are soldered, if one crystal becomes defective, the stack of crystals must be replaced. The spring contacts against the radially inward surfaces of the crystals may also scratch the silver plate on those surfaces, possibly causing the quality of the electrical contact at those points to deteriorate.
It is desirable to provide a noise logging tool with a sound sensing device including electrical conductors which are efficient and relatively easy to install and maintain, which provide good electrical connections with the crystals, and which do not cause any deterioration of the quality of those electrical contacts.